Nobel Prize in Physics: what is the second quantum revolution?

Finally ! The Nobel Prize in Physics was awarded to three pioneers of the second quantum revolution: the Austrian Anton Zeilingerthe American John Clauser and French Alain Aspect, For their “experiments with entangled photons, establishing the violation of Bell’s inequalities and paving the way for quantum information science”. Each of them, says the press release “conducted groundbreaking experiments using entangled quantum states, where two particles behave as a single unit, even when separated”.

The only thing clear in this press release, it was time, and for a long time, to reward these men who learned to manipulate photons and electrons one by one, to observe behavior so disconcerting that no one, be it a physicist or a philosopher, still knows today. today how to deal with it.

A little throwback, in three steps at no charge.

1) The first revolution: the Einstein-Bohr duel

The first revolution took place between 1900 and 1930. The Glorious Thirties of Physics begin with Max Planckcontinue with Albert Einstein, Niels Bohr, Werner Heisenberg and Erwin Schrödinger. The revolution they initiate is based on the famous wave-particle duality advanced by Louis De Broglie : the objects of microphysics sometimes behave like waves, sometimes like particles – it all depends on the measuring instrument. How to interpret this duality? Bohr and Einstein will clash around this question by defending fundamentally different interpretations, Bohr accepting without flinching a probabilistic interpretation of the world – a quantum measurement does not give a result but the probability of this result –, Einstein remaining convinced that a good theory must give a result accurate for every situation. Otherwise, it is incomplete.

In support of his thesis, there was a catch, brilliantly underlined with Boris Podolsky and Nathan Rosen in 1935 in an article that has remained famous under the name of ERP paradox : Einstein and his collaborators explained that the formalism of quantum mechanics makes it possible to imagine a pair of particles in a strange state: a state “entangled”which means that two particles having interacted at the beginning of an experiment can be separated as much as we want, or can – say one staying on Earth and the other going to Mars – (later experiments were carried out over distances from meters to kilometers), they remain correlated ; the measurement made on one instantly results in the result of the measurement made on the other. This is what Einstein called a “remote phantom action”. Why ghost? Because he didn’t believe it. In classical physics, the principle of locality (or separability) supposes that two distant objects cannot influence each other instantaneously, otherwise, he thought, we fall into sorcery: a physical magnitude, in his eyes, must be able to be defined locally, at each point of the space, therefore separated from what is happening elsewhere.

Bohr’s interpretation will still win the song, basically because quantum mechanics works ! The transistor, the integrated circuits, the laser… Everything that will found the technology of information science derives from it and arises year after year from the quantum cornucopia. Faced with such success, Einstein’s objections are placed in the oddities and artifacts department.

2) The second revolution: proven “non-locality”

In the 1970s, the second quantum revolution occurred quietly. Those who are interested in the foundations of the theory are few: a few years ago, Zeilinger reminded one of us (Jean-Pierre Pharabod) that at that time, physicists who were interested in the foundations of quantum mechanics were discredited. ” In my eyesremarked Zeilinger, it was a case of collective cognitive repression in the sense of Piaget. When I was a young post-doc and gave my first talks, the senior physicists were often so hostile that I had taken to talking to the young people at the back of the room who were passionate. »

However, the progress made in electronics then offer the possibility of manipulating atoms and electrons one by one. And that changes everything. In the early 1960s, Irish (Northern) John Bell had proposed a test which would make it possible to decide which, of Bohr or Einstein, is right. Unrealizable on the experimental level at that time, his proposal did not have any enthusiasm. But the situation has changed. In 1972, the Americans John Clauser and Stuart Freedman put together an experiment that schematizes Bell’s test, but it is too imprecise. It will be Alain Aspectin his laboratory at the Institut d’Optique d’Orsay, which will mount the decisive experiment, in 1982. He demonstrates experimentally the apparent non-locality of quantum entanglement, that is to say the property which translates the capacity of two photons to seem to exchange information, even at a distance (however considerable) from each other.

3) The quantum scene today

Forty years later, thanks in particular to Zeilinger’s experiments, non-locality is a proven fact. And the quantum horn is pouring out a second revolution, of which we are now seeing the first fallout through the quantum cryptography and soon the quantum computer.

There remains a huge philosophical question mark: the mathematical formalism of quantum mechanics is well marked out, but the concepts remain more slippery than fish landed from a trawl. These entangled particles, which seem to form a whole even though they are, putatively, light years apart, form systems with which everyone, physicist or philosopher, manages as best they can.

But the fact is that no one can offer entirely satisfactory explanations. Everything happens as if these pairs of particles were located outside of space-time. Or as if words were insufficient, unsuited to describe the quantum world, a world in which it is the measuring device that determines the properties of the object. Enough to fuel a third revolution: “Quantum physicswrites Alain Aspect, is extraordinary, insofar as, after a century of effort, no phenomenon is known where it seems to reach its limitss. »